1. Field of the Invention
The invention is concerned with speech coding, and more particularly to systems in which a speech signal can be generated by feeding the output of an excitation source through a synthesis filter. The coding problem then becomes one of generating, from input speech, the necessary excitation and filter parameters. LPC (linear predictive coding) parameters for the filter can be derived using well-established techniques, and the present invention is concerned with the excitation source.
2. Related Art
Systems in which a voiced/unvoiced decision on the input speech is made to switch between a noise source and a repetitive pulse source tend to give the speech output an unnatural quality, and it has been proposed to employ a single "multipulse" excitation source in which a sequence of pulses is generated, no prior assumptions being made as to the nature of the sequence. It is found that, with this method, only a few pulses (say 8 in a 10 ms frame) are sufficient for obtaining reasonable results. See B S Atal and J R Remde: "A New Model of LPC Excitation for producing Natural-sounding Speech at Low Bit Rates", Proc. IEEE ICASSP, Paris, pp. 614, 1982.